A large amount of hexavalent chromium-containing residues can be produced in the production process in an industry of production and application of chromium salt. During transport and storage of chromium-containing residues, a large number of hexavalent chromium would be released gradually to the environment if the chromium-containing residues are subjected to weathering and soaking of rainwater for a long period, and it not only makes a serious pollution to the surroundings but also wastes a great deal of metal chromium resource.
At present, a core concept for the treatment method aiming at hexavalent chromium-containing residues is stockpiling or landfill after reductive immobilization of hexavalent chromium, mainly including wet detoxification, dry detoxification and biological treatment. For the wet detoxification, hexavalent chromium in the chromium-containing residues is leached into water phase by alkali or acid, and hexavalent chromium is reduced to trivalent chromium by adding a reductant to achieve detoxification. However, the reduced trivalent chromium will be oxidized to hexavalent chromium as the surroundings change, with incomplete detoxification, and it is difficult to apply the treated residues and takes lots of lands for stockpiling. For the dry detoxification, by calcination in a high-temperature reducing atmosphere, hexavalent chromium is reduced to trivalent chromium and enclosed in a sinter to achieve the purpose of detoxification, but such method needs a relatively high detoxification temperature (generally above 800 degrees) and the exhaust gas produced by calcination would generate a secondary pollution. It demands addition of an apparatus of removing smoke and dust, which results in an increased cost. The biological treatment has received more and more attention due to its advantages such as non-pollution and low cost. However, it still has several drawbacks, for example long activation time, low treating speed, and high consumption of organics and so on.
The methods described above only reduce and immobilize hexavalent chromium, and still chromium would be discharged to the environment. Not only the chromium resource cannot be recycled or utilized, but also the secondary pollution such as “yellowing” generates. Therefore, a relatively ideal method is to extract chromium from the chromium-containing residues and to reuse it in the industry, reducing the discharge of chromium from the source, turning waste into treasure, turning harm into good and achieving resources as the pollution is reduced.
CN patent “method of recycling hexavalent chromium resource from chromium-containing residues” (CN102191390A) collects the chromium resource by leaching water-soluble and acid-soluble hexavalent chromium with sulfuric acid or hydrochloric acid at normal temperature prior to reduction and precipitation. Although a recovery of chromium from the chromium-containing residues in such method can reach 60%-80%, part of hexavalent chromium still remains in the treated residues. Detoxification of chromium-containing residues is incomplete and it is difficult to comprehensively utilize the treated residues with an environmental hidden danger remained. CN patent “process for processing, recycling and reusing chromium-containing residues and effluent” (CN102699006A) nearly completely dissolves the chromium-containing residues with acid at normal temperature, then adds different precipitants respectively, and precipitates and separates mixed ions in the solution. Such method demands a big consumption of chemical reagents such as acid and precipitant, with complicated process and relatively high treatment cost.